The invention relates to an active pressure intensifier which comprises an axial piston pump having a housing in which a drum which is driven via a drive shaft for rotation and has at least two piston chambers is disposed, the piston chambers respectively having a liquid inlet and a liquid outlet and, in the piston chambers respectively, a piston with at least one piston rod being disposed. Likewise, a reverse osmosis plant which has this active pressure intensifier is provided according to the invention. Likewise, a method for changing the concentration of dissolved components in liquid solutions by means of reverse osmosis is provided according to the invention. The subject according to the invention is used in particular in sea- and brackish water desalination, in waste water treatment, in the foodstuff industry, in the chemical industry and in mining.
There is a large number of high-pressure applications in which energy recovery from the potential energy of a partial volume flow appears sensible. In the fields of reverse osmosis, mining and chemical processing technology, this sort of thing is partially achieved. For all these applications, it is possible to use the invention. The basic principle is explained with the example of the reverse osmosis process for sea water desalination.
In general, in the case of such plants, sea-, brackish- or salinated well water is forced as so-called feed or supply into one or more membrane modules at high pressure above the osmotic pressure. The feed volume flow is divided inside these membrane modules into two partial volume flows, consisting of permeate volume flow and concentrate volume flow.
The permeate volume flow emerges quasi-unpressurised from the membrane and, in the case of sea water desalination, represents the product of the process which can be used subsequently as fresh water. In the case of other reverse osmosis processes, in which concentration of fruit juices for example is of concern, the concentrate mass flow forms the desired product mass flow. The ratio of permeate volume flow to feed volume flow is thereby defined as output rate. In the concentrate volume flow, consisting of concentrated salt water in the case of sea water desalination, potential energy is stored on the basis of its high pressure. The use of this high pressure forms the approach for all known systems of energy recovery in reverse osmosis plants. Energy recovery can basically be effected in two ways: by conversion or direct transmission of the concentrate pressure. In the case of pressure transmission, there are possibilities of isobaric or pressure-intensifying exchange.
In the case of conversion of the concentrate pressure, the energy stored at high pressure of the concentrate can be converted, with the help of flow- or displacement machines, into mechanical energy and subsequently can be used to assist the pressure increase at the beginning of the process. In the case of displacement machines, the output rate is fixed by the stroke- and suction volumes of pump and motor. An example of this is a combination of axial piston pump and -motor. Pump and motor are connected to each other via a shaft, as a result of which a centrally disposed electric motor is relieved of load during driving of the pump. Flow machines, on the other hand, allow in fact in general a variable output rate as a function of a plant characteristic but, because of their kinematic properties, can only be used efficiently in reverse osmosis plants of a fairly large construction.
In the case of conversion of the energy stored in the concentrate pressure for mechanical relief of load of the feed pump, few improvement options result since the components are already optimised with respect to flow technology and the only approach would reside in reducing mechanical loss factors. However the potential is estimated to be low.
As an alternative to energy conversion, the concentrate pressure can also be transmitted directly to the feed volume flow. These two volume flows must not however mix notably during the pressure transmission since, otherwise, because of the increased salt concentration in the feed, the osmotic pressure thereof and ultimately the required plant pressure would rise. For this reason, a piston cylinder is used in most systems for pressure exchange, as a result of which the volumes with a different material concentration are separated spatially.
If the pressure working surfaces on both sides of the piston are identical, for example in the case of pressure exchanger pipes with rodless pistons, then this is termed isobaric pressure exchange. Here, the pressure is conveyed without hydraulic transmission from the concentrate to a part of the feed. Plants of such a construction are operated in cooperation with a high-pressure pump (HPP) and a recirculation pump (RCP). FIG. 1 shows a plant according to this plan. An HPP 1 forces feed 2 in the direction of one or more membrane modules 3 by effecting a separation of the volume flow into the permeate volume flow 4 and the concentrate volume flow 5. The concentrate 5 is subsequently conducted via a valve arrangement 6 into a first of a plurality of pressure exchanger pipes 7, 8 in which it displaces a piston 9. On the other side of this piston 9, feed 10 is received in this first pressure exchanger pipe and is discharged by displacement in the direction of the membrane module 3.
The continuous operation of the plant is ensured by a large number of pipes which are supplied intermittently with concentrate via a valve arrangement 6. Whilst a first pipe is filled with concentrate, the now quasi-unpressurised concentrate from the previous cycle must be discharged out of a second pipe and this must be filled with fresh feed for the following cycle. For this purpose, a low-pressure filling pump (LPP) 11 is required. During switch-over of the valves, the result can be at times strong pressure surges in the system since the concentrate volume flow comes to a standstill for a short time on the high-pressure side. Valve concepts which prevent this are very complex and not obtainable in small constructional sizes.
Since the concentrate loses pressure as a result of friction when passing through the process, this pressure loss must be compensated for before feeding the feed in again into the volume flow from the HPP 1 by means of an RCP 12 since otherwise no circulation would be able to be maintained. The RCP 12 represents a disproportionate cost factor because housing and sealing elements must be designed for high pressure although only a low output of the recirculation flow need be accomplished. This leads to a niche product which increases the initial costs.
By means of separate control of HPP 1 and RCP 12, the volume flows of feed and concentrate can be specifically controlled in the case of the energy recovery concept of the isobaric pressure exchange. Consequently, the great advantage of a variable output rate is presented. Particularly in the case of a varying salt content of the feed and also in the case of a varying supply of energy, this is of interest since any number of plant operation points can be controlled due to the variable output rate. Hence, an energy optimum of the desalination process can be achieved at any time. Because of the relatively complex construction of such plants, this concept has to date generally only been applied in large plants.
If the pressure working surfaces on both sides of a piston are not identical, then the result is a pressure change over the piston due to a hydraulic transmission. The piston is operated together by a pump and the concentrate pressure.
This represents the approach of so-called pressure intensification and leads to small, compact systems which are simpler with respect to the plant construction. Conveyance of the feed and also the energy recovery can be effected in a single component. Analogously to the displacement machines in energy conversion, the ratio of the volume flows of feed and concentrate, and hence the output rate, is always however fixed constructionally in the case of pressure intensifiers. In the case of constant operating conditions, this does not represent a disadvantage because the plant can be designed correspondingly for the nominal operating point. With respect to variable operating conditions, such systems can however barely be adapted.
By way of clarification, it must be said in addition that, in the case of previous intensifying pressure exchange, merely intensification of the initial pressure of a preceding pump is achieved with the aid of the concentrate pressure or a driving motor is relieved of load. As a result of the hydraulic transmission from piston ring surface to piston surface as such, the pressure transmitted from the concentrate to the feed is even on the contrary de facto reduced since the pressure working surface on the part of the feed is greater than on the part of the concentrate.
A system is known from U.S. Pat. No. 7,799,221 B1. This is based on the principle of transmission of the concentrate pressure to the feed. This is hereby achieved by a swash plate axial piston pump (alternative embodiments as inclined axes or wobble plates).
In contrast to known pumps of this construction, no finger pistons are used, but rather piston and piston rod with different cross-sections. The pump has a relatively simple construction since it consists of few moving components. Two oppositely situated axially flush end plates are connected together via a housing in which a drum (driven by an electric motor) rotates with a large number of piston chambers distributed axially over the cross-section.
On the side of the piston surfaces there is the end plate which enables the entry and exit of the feed. On the side of the piston ring surfaces there is the end plate which enables the entry and exit of the concentrate. The ends of the piston rods are provided with feet which slide over a stationary swash plate. The concentrate under high pressure is conducted to the piston ring surface of a cylinder. The feed has already been received on the side of the piston surface at this time. The electric motor which drives the pistons by means of inclination of the stationary swash plate is consequently relieved of load by the concentrate pressure.
As a function of the angle of rotation of the drum, the pistons are either driven forward, concentrate under pressure being conducted towards the piston ring surface and the received feed being discharged in the direction of the membrane module, or moved back, new feed being received and the quasi-unpressurised concentrate being ejected. Since the piston rods are not connected rigidly to the swash plate, the feed must be at slight high pressure during inflow so that the piston can be moved back and the concentrate can be ejected. Hence, a further (mentioned only indirectly in this concept) filling pump is required. Otherwise, the pistons would remain in the front position.
It is striking in this system that guidance of the volume flows is possible without valve arrangements. Above the surfaces of the end plates, radial passages which have cross-sections which are optimised with respect to flow technology are disposed. On the concentrate-side, arcuate oblong holes are inserted so that the inflow into a piston chamber is gradually released and closed, whilst, in a subsequent piston chamber, the filling with concentrate can already begin. As a result, pressure surges during operation can be avoided ideally.
A previous approach of the Fraunhofer Institute for Solar Energy Systems, for combining a compact plant construction with the possibility of a variable output rate, is known from the unexamined German application DE 10 2009 020 932 A1. However, large drive forces and irregular dimensions of the required semi-finished product raise questions about its use.